Conventional coating materials which can form permanent films for the surface protection of steel plates such as stainless steel plates, non-ferrous metals such as aluminum, inorganic construction materials such as concrete and slate, plastic substrates, and the like include coating materials obtained by the hydrolysis or partial hydrolysis of hydrolyzable organosilanes and coating compositions obtained by mixing a colloidal silica with the above coating materials.
For example, JP-A-51-2736, JP-A-51-2737, JP-A-53-130732, and JP-A-63-168470 propose coating materials which are obtained from an organoalkoxysilane, a hydrolyzate of the organoalkoxysilane and/or a partial condensate of the hydrolyzate, and a colloidal silica, and in which the alkoxy groups have been converted to silanols by using an excess of water. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) These coating materials give excellent protective films having high hardness and good weatherability. However, they have a disadvantage that since baking or heat treatment should be conducted at a temperature as high as 100.degree. C. or more or for a prolonged period of time in order to obtain desired film properties, there are cases where these coating materials are not applicable depending on the molding method for the substrate, the size and heat resistance of the substrate, or whether the coating materials are used outdoors or not. In addition, the above-proposed coating compositions have a problem that they have poor stability and tend to gel because the reactivity of the silanols formed by the hydrolysis of the alkoxysilane is so high that the silanols gradually undergo a condensation reaction even at ordinary temperature. A further defect of these coating compositions is that they cannot form pigment-containing coating paints because if a pigment is added to such coating compositions as a vehicle, the poor stability of the coating compositions becomes even worse.
JP-A-64-168 proposes a coating material comprising a partial condensation and partial hydrolysis product of an alkoxysilane and a hardener which is water and a catalyst, the hardener being added to the partial condensation and hydrolysis product just before use thereby to convert the alkoxy groups to silanols. Although such a coating material has good storage stability and is relatively stable even after a pigment is added thereto, it is disadvantageous in that baking or heat treatment should be conducted at a temperature as high as 100.degree. C. or more or for a prolonged period of time in order to obtain desired film properties and, hence, there are cases where the coating material is not applicable depending on the shaping method for the substrate, the size and heat resistance of the substrate, or whether the coating material is used outdoors or not.
For the purpose of eliminating the above problems, JP-A-63-268772 proposes a coating material which comprises a prepolymer mainly comprising silicon alkoxide, a curing catalyst, and water, and cures around ordinary temperature. However, this coating material is defective in that it has poor coating properties and the curing properties thereof are poor and tend to be affected by moisture.
On the other hand, it is known to use silicone resins as a vehicle for heat-resistant or weather-resistant coating compositions. Most of these silicone resins comprise silanol group-containing polysiloxanes. In general, such silanol group-containing organopolysiloxanes are produced through hydrolysis of organochlorosilanes and are available as solutions in toluene or xylene. In the case of using organoalkoxysilanes as the raw material, silanol groups undergo condensation reactions until the hydrolyzates of the organoalkoxysilanes become soluble in toluene or xylene. The thus-obtained silicone resin solutions show good stability even after pigments are kneaded therewith to give coating fluids. However, they are disadvantageous in that formation of cured films necessitates high temperature and prolonged heat treatment and that a film hardness attainable even by such prolonged heat treatment is limited and insufficient for permanent films.
The above-described inorganic hardened products such as concrete and other cement-derived products are materials having excellent heat resistance and durability, but they are defective in that water penetrates into the inorganic materials if the surfaces thereof are left uncoated, and that they are poor in resistance to stains and acids. Further, the uncoated surfaces of these inorganic hardened products are not good in appearance.
Coating the surfaces of such inorganic products with organic coating compositions has been conducted to overcome the above problems. However, the organic coating compositions are disadvantageous in that they have poor weatherability and films formed therefrom tend to suffer scratches due to their low hardness.
In place of the organic coating compositions, it has been proposed to use inorganic coating materials such as waterglass-based coating materials. However, satisfactory results could not be obtained with respect to the occurrence of efflorescence and porosity.
Silicon alkoxide-based coating materials are studied as inorganic coating materials free of the above-described defects. However, use of silicon alkoxide-based coating materials has the problem of alkali generation from the cement-derived hardened product used as the substrate. Further, there is a problem that the coating tends to suffer cracking or peeling due to rain water and temperature change because the substrate shows a relatively large dimensional change and the adhesion of the coating to the substrate is insufficient. Although application of silicon alkoxide-based coating materials through primer coatings is proposed in, for example, JP-A-63-262203 and JP-A-1-83580, the silicon alkoxide-based coating material applied on the primer layer should be heat-treated at 100.degree. C. or a higher temperature and the time period for this heat treatment is considerably long. Therefore, application of these coating materials at construction sites or application thereof on substrates having insufficient heat resistance had the tendency to cause occurrence of cracking during heat treatment. This cracking problem has also accompanied cement-derived hardened substrates if they have a tongue, a projection-depression pattern, or the like and have a small-thickness portion therein.